In the context of “multicellular cooperative” communications, terminals receive payload signals from a plurality of base stations cooperating with one another.
This technique is very promising but is still open to improvement, notably to simplify its use.
Multicellular cooperative communications enable terminals to receive payload signals from a plurality of base stations in a coordinated manner that advantageously minimizes interference between cells. Intercellular interference is the main reason for the limited spectral efficiency of cellular systems. Provision is made for a few base stations that exchange information efficiently to cooperate with one another. The cooperating base stations form what is known as a cooperation cluster.
Apart from reducing intercellular interference, transmission by a plurality of base stations also offers advantages in terms of robustness against:                shadowing; and        fast fading type signal losses;because the transmission channels between a terminal and different base stations need not be correlated. This is referred to as macro-diversity.        
Multicellular cooperation is a multiple-input multiple-output (MIMO) multi-user technique enabling the base stations of the cooperation cluster to act as a single array of distributed antennas. One of the most promising uses of this distributed array with a view to increasing the overall data rate of the cell is providing space-division multiple access (SDMA), i.e. servicing a plurality of users simultaneously using the same block of resources. A block of resources represents the unit quantity of radio resources that a user may be allocated.
For example, in the code-division multiple access (CDMA) technology, a block of resources may be defined by:                a carrier frequency;        a spreading code; and        a duration.        
In orthogonal frequency-division multiple access (OFDMA) technology, a block of resources may be a set of subcarriers during a set of orthogonal frequency-division modulation (OFDM) symbols, known as a chunk.
Macro-diversity has already been used for a few years by Universal Mobile Telecommunications System (UMTS) technology to improve performance for users at the edges of cells. UMTS technology does not use SDMA, however: the base stations that cooperate transmit to only one user at a time on one frequency and using a given code.
Linear pre-coding (using adaptive beams) and the edge information insertion technique known as dirty paper coding are two techniques for implementing SDMA on the downlink (from the station to the terminal), as opposed to the uplink (from the terminal to the base station).
Linear precoding may also be effected on the basis of fixed beams (this is known as the grid-of-beams technique). Linear precoding is a transmission technique that may prove advantageous in practice because it represents a good compromise between performance and complexity. In each transmission time interval (TTI), the base stations of the cooperation cluster may service at most as many terminals for each block of resources as the total number of antennas in the cluster. The TTI is the unit time interval during which a user may be assigned radio resources. SDMA may be used on the downlink if the base stations have channel state information (CSI) in respect of the transmission channels on the downlink between the antennas of the base stations and the antennas of each user. This CSI may consist of the coefficients of the transmission channels, for example.
However, linear precoding may also be used in the context of multicellular cooperative communications without SDMA, a single terminal then being serviced in each TTI. The user being serviced then benefits from more power and therefore improved received signal quality, but the overall capacity of the system is generally low compared to the situation in which SDMA is used. Servicing a single terminal at a time may nevertheless be preferred if the requirement is to provide a good quality of service to some users suffering particularly unfavorable reception conditions.
In the fixed beam situation, the CSI may equally be information in respect of the beam that is the most appropriate one for the terminal, i.e. the beam that is received with the maximum power.
Moreover, packet communications systems generally require information about the quality of the channel to be made available in order to determine the appropriate combination of modulation and coding to use for transmission. This information is traditionally referred to as the channel quality indicator (CQI) and may be a signal to interference plus noise, for example. Moreover, this CQI may also be used to determine the scheduling of the terminals to be serviced.
In the present document, the general term channel information (CI) is used to designate the combination consisting of the CSI and the CQI. Below, the expression “local CI”, when used with reference to a base station, refers to the CI on the transmission channels between a terminal and the antennas of that base station. The CI that is not local to a base station is the CI on the transmission channels between a terminal and a different base station.
In the context of multicellular cooperative communications on the downlink of frequency-division duplex (FDD) systems, the terminal must estimate the CI of the transmission channels between its antennas and the antennas of the base stations of the cooperation cluster with which it is associated. The CI must then be transmitted to a central entity known as the central controller (CC).
The CI may be transmitted from the terminal to the central controller using the following method: the terminal transmits over the return path to each base station of the cluster the CI local to that base station. Note that in this method a return path is necessary for each base station. Each base station then transfers the received CI to the central controller.
The central controller assembles the CI from all the terminals of the cluster and decides on the users to be serviced. This process is referred to as scheduling. The central controller also defines the transmission parameters to use on the basis of the CI from all the terminals of the cluster, for example a precoding matrix to use for linear precoding or the beam to activate for fixed beams.
The existence of a central controller for each cluster and the exchange of information between the base stations and the central controller are necessary because in the two methods referred to above each base station knows only a portion of the CI of the terminals necessary for scheduling and to define the transmission parameters.
In the prior art described in particular in the document “Collaborative MIMO Based on Multiple Base Station Coordination”, Y. Song, L. Cai, K. Wu, and H. Yang, Contribution to IEEE 802.16m, IEEE C802.16m-07/162, multicellular cooperative communications are implemented in three stages.
In a first stage, each terminal estimates the transmission channels between its antennas and the antennas of each of the base stations of the cooperation cluster. For example, if each terminal has only one antenna and if there are B cooperating base stations, each of which has two antennas, each terminal estimates 2xB CSI. Similarly, each terminal estimates a CQI for each base station.
In a second stage, the terminals transmit on the return path of each base station of the cluster the CSI and CQI local to that base station using on each return path an appropriate power and an appropriate modulation and coding scheme for the base station to be able to decode the message. Note that, because they depend on the quality of the channel on the return path, the power and the modulation and coding scheme may be different on each return path. Consequently each base station of the cluster assembles local CI.
The base stations finally send the local CI to the central controller of the cluster.
In a third stage, the central controller decides on the scheduling of the users. The central controller then decides on the transmission parameters to use. It calculates a linear precoding matrix, for example. Finally, the central controller sends the information corresponding to its decisions to each base station of the cluster.
Note that in the time-division duplex (TDD) situation, as considered in the document referred to above, the first stage and the operation of sending the local CSI of the second stage are simplified because a base station may estimate the CSI of the mobiles in its cell directly. In the TDD situation, the uplink channel is identical to the downlink channel (in accordance with the principle of channel reciprocity). However, all other operations are unchanged.
Moreover, note that the document referred to above proposes to simplify multicellular cooperative communications by limiting the exchange of CI between base stations to the CQI, each base station forming a linear precoding matrix exclusively according to its local CSI. This reduction of the exchanges necessary between the base stations and the central controller simplifies the system, but at the cost of degraded performance as the intercellular interference then cannot be treated effectively.
The prior art technique, even when simplified as in the document referred to above, requires infrastructure cost that is high relative to the existing structure of cellular systems because it requires a central controller for each cooperation cluster and low-latency links between the base stations and the central controller. Furthermore, it is necessary to define new protocols in order for the cooperation cluster entities (central controller and base stations) to interwork correctly, in particular to coordinate:                the exchange between the base stations and the central controller of local CI (second stage);        information on the users selected by a scheduling unit (third stage); and        information concerning the transmission parameters (third stage).        